No sculpting shown in sculpt mode
What?

You seem to be editing a shape key (also known by some as a morph target) that has zero weight. You can find shape keys in Properties > Object Data > Shapekeys. A shape key - grossly simplifying - is a snapshot of the arrangement of vertices in a mesh. With shape keys it's possible to interpolate between different states of the mesh.

How to fix?

The active shape key is the one that is being modified by the sculpting, but it may have weight zero and thus not be reflected onto the mesh. To pick which key is being edited, simply select it from the list. While sculpting you may wish to activate the pin icon: it will mute all but the active key. Just remember to deactivate it when you start rigging if you want to avoid surprises.

Why is this a thing?

Deforming a character with bones alone often leads to unsightly wrinkles. To combat this, many character rigs (including those made by Makehuman) are designed with shape keys that make the folding of elbows and  knees more natural and that are triggered automatically by bone motion. Another related use is driving creasing in clothing or bulging muscles. Shape keys (here the name morph target is somewhat more fitting) can also be used to drive radical but gradual transformations from one character to another.